1. Field of the Invention
The invention relates in general to the field of polymeric composites that can be used as construction materials. Specifically, the invention relates to polymeric composites having a polymeric component that is preferably olefin and preferably recycled, a rubbery polymeric component, and a reinforcing filler component containing mica.
2. Description of the Related Art
The use of polymeric materials in commerce has increased steadily since the introduction of some of the first synthetic polymers such as Bakelite. On a volume basis, polyolefins, including notably polyethylene and polypropylene, are one of the most widely produced families of polymers. Their use pervades numerous industries, including both thin and thick walled packaging, containers, toys, wire and cable jacketing, automotive parts, and medical supplies.
One of the benefits of the use of plastic or polymeric materials is the unique combination of light weight and strength available with them. The chemical, electrical, physical, and other properties of polymeric materials can be somewhat modified to meet the performance criteria for different products. Another benefit to plastic materials is their ease of manufacture by both molding and extrusion processes.
Polymeric materials are additionally not readily biodegradable. Because of this, consumer and industrial articles formed from polymers can have a much longer effective lifespan than comparable natural materials. A majority of polymeric material will often contain various stabilizers including both antioxidants and UV stabilizers to further extend their useful life. This long lifespan is however also one of the more negative aspects incumbent with the use of polymers. The fact that a very large proportion of polymers and in particular polyolefins are used in disposable or short-lived applications necessitates that a considerable amount of waste polymer is generated shortly after it is produced. Until recently a very large proportion of this waste plastic found its way into landfills. The use of these waste plastics as a recycled component in products has recently led to marginal reductions in the amount of waste plastic; however, the waste material used in this manner is often incorporated into articles which themselves have a relatively short lifespan. A more effective long term solution to the growing volume of waste polymer, particularly polyolefins, would be to utilize the waste plastic as a component in construction materials that require a relatively long lifespan. Exemplary long use construction materials would include railroad ties, parking curbs, marine pilings, decking or other structures in docks, and numerous others. When polymers are used for these purposes, they are typically combined with various other ingredients, such as reinforcing fillers. Polymeric compositions containing a reinforcing filler and which may contain other components are commonly referred to as composites.
These composite materials can be particularly beneficial when they are used to replace wood. For example wood based railroad ties are particularly susceptible to wear and deterioration due to processes such as erosion. In environments in which the railroad tie is subjected to numerous cycles from freezing to non-freezing conditions, the tie will crack as water which has penetrated the tie freezes and expands. Additionally wooden ties are subject to insect attack when creosote treatment is not optimal or when the creosote leaches out of the tie. Conversely, comparable materials formed of polymer containing composites are not as susceptible to water penetration. Additionally, the composite can more effectively distribute the stress resulting from that water that does penetrate, freeze, and expand. The ability to distribute the stress results in less cracking and warpage. The composite railroad ties are also not susceptible to insect attack.
The waste polyolefins which are typically utilized in these construction materials or other rigid structural members may contain high density polyethylene (HDPE), low density polyethylene (LDPE), linear low density polyethylene (LLDPE), other polyethylenes, polypropylene including both homopolymer and copolymer variations (e.g., propylene- ethylene copolymers), and combinations of these polymers. Unlike virgin polymers, these waste polymers have been subjected to at least one heat processing step and often usually have been exposed to environmental conditions, often for extended periods of time. Because of this, these waste polymers have very different properties than their virgin counterparts. The waste polymers will typically have lower flexural and tensile strength and lower thermal stability than virgin materials.
The waste polyolefin component of construction materials may often additionally contain minor amounts, typically less than about 20%, of various other polymers including polyvinyl chloride (PVC); chlorinated polyethylene (CPE); chloro-sulfonated polyethylene; various compounded polymers; polystyrene; and various engineering thermoplastics such as polyamides, polycarbonates, thermoplastic polyesters, and ABS.
Construction materials (or articles) formed of composites containing these waste polymers will typically contain a rubbery, typically polymeric, material to add impact strength and flexibility to the construction article. Any number of polymeric materials may be utilized for this rubbery component, including natural rubber, EPDM, styrene butadiene rubber, and styrene butadiene styrene rubber. However, in keeping with the goal of reducing the volume of waste deposited in landfills, a convenient source of the rubbery component is tires. Tires typically contain rubber; steel; and polyester or other strands or fibers. Various machines known in the art can be utilized to cut, grind or shred tires into tiny fragments that can then be utilized in processes which can form the desired composite material.
Another component that is typically found in these composite articles is a foaming agent which is used to control the density of the composite article. A typical foaming agent system will contain a Group I metal (alkali) bicarbonate and a bicarbonate salt of a saturated fatty acid. Alkali metal salts employed include sodium and potassium bicarbonate, while suitable saturated fatty acids include those having from 14 to 22 carbon atoms. The two compounds react together releasing CO.sub.2 which forms voids in the solidified composite. The voids reduce the density of the final composite article, thus reducing the amount of raw materials required for a given volume of article, while at the same time increasing the strength to weight ratio of the composite article. The use of these foaming agents can however dramatically increase the cost of the composites. Additionally, because these foaming agents are utilized in such small amounts, typically less than about 2.0% of the total composite mass, a homogeneous distribution of the foaming agent is difficult to achieve. As a result, a uniform distribution of the voids within the composite article is difficult to achieve. The non-uniform distribution of voids results in a non-uniform distribution of weight within the article and non-uniform physical properties.
Finally, the addition to the composite of a reinforcing filler, depending upon its morphology and other properties, may enhance the tensile strength, impact strength, stiffness, and heat distortion properties of the composite. The reinforcing fillers are often used in conjunction with coupling agents, such as silanes and titanates, to effectuate the incorporation of the filler into the polymer matrix. Reinforcing fillers that have been used for these purposes in various composite structures include fiberglass, asbestos, wollastonite, whiskers, carbon filaments, talc, kaolin and other clays, mica, calcium carbonate, fly ash, and ceramics. Filamentous fillers such as glass fibers typically provide the greatest impact and tensile strength properties while the addition of more platy structures like talc and mica typically result in increased stiffness and heat distortion. A single filler or multiple fillers may be used depending upon the desired properties. Glass fibers are in particular commonly used as a reinforcing filler in composites because it is known that the glass fiber will generally improve stiffness without significantly reducing impact properties or increasing density. However, glass fibers are typically the most cost prohibitive reinforcing filler and additionally result in significant wear to processing equipment. As a result, less expensive fillers, such as talc and mica, have been used to replace either part or all of the glass fibers in a composite. Unfortunately, these fillers usually have a much higher density which has heretofore resulted in heavier composites article than those employing glass fibers. It would be beneficial if composites containing these or other inorganic fillers rather than or as a substitution for a significant portion of glass fiber could be devised that did not have the density concerns previously indicated.
In addition to the above described components, composites may contain other additives depending upon the intended use of the article. Compatibilizers are often use to effectuate the mixing (i.e., compatibility) of two or more polymers which might comprise the source of polymer used in the composite. These compatibilizers will typically have reactive groups that upon heating and shearing will react with the polymers via free radical or ionic mechanisms. Compatibilizers which have been employed include the various maleic anhydride copolymers and ionomers, acrylate copolymers, and ethylene acrylic acid copolymers. The composite may additionally contain antioxidants, UV stabilizers, lubricants, antifungal agents, and colorants. These various additives may be added during fabrication of the construction article or may be present in one of the initial polymeric components.